This invention relates to analog signal amplification and particularly to high-efficiency power amplifiers.
A signal amplifier draws power from a fixed power supply, Vdd commonly referred to as a rail, which is provided by a power source such as battery or a battery followed by a voltage regulator provided for voltage stability. For portable amplifiers, the efficiency of power drawn from the source is very important since an inefficient usage of power can result in a rapid drain of the battery resulting in short operating times between recharging or replacement. The ratio of the power delivered to the load Pload, to the power drawn from the battery (Pbatt) is the measure of efficiency of the signal amplifier.ξ=Pload/Pbatt  (1)
Some types of amplification techniques (e.g., class D) that have been used to increase efficiency employ a switching device as the amplifier. The switching device typically places constraints on the type of signals for which such amplifiers can be used since the device operation is non-linear. However, the present invention is directed to classes of amplifiers for use for both linear and non-linear signal amplification. Class G amplifiers can be used for both linear and nonlinear applications. Class G amplifiers employ several amplifiers in parallel that operate off of different rail voltages, each of which contribute varying amounts of power to the load depending on the signal level. Such amplifiers are more efficient in power delivery and can be used for linear signal amplification. Class G amplifiers can approach 80-90% peak efficiency compared to class AB amplifiers (64% peak efficiency). In addition, they offer the benefit of better efficiencies at lower power levels, which is important where signals have high peak-to-average ratios.
Conventional implementation of class-G amplifiers fixes the number of parallel amplifiers and operating rail. The voltage rails (VRi, i=amplifier instance) required by the parallel amplifiers are usually provided through separate external power sources or are generated using a single power source employing reactive components (capacitor or inductor) as intermediate power stores for power delivery as required. A capacitive charge pump is one such power store. Reference is made to U.S. Pat. Nos. 7,061,327, 7,061,328, and 7,183,857 for background. The efficiency of a class-G amplifier depends on the number of rails as well as the input signal statistics, such as peak-to-average ratio. The actual value depends on the difference between the rail voltage and the signal threshold at which transitions between different amplifiers occur. Theoretical efficiency of a class-G amplifier approaches 80-90% independent of the load power when the number of rails approaches infinity. However, it is impractical to have large numbers of rails.